Instabilities of Spiral Shocks – Ii. a Quasi-steady State in the Multi-phase Inhomogeneous Ism

The “galactic shocks” (Fujimoto 1968; Roberts 1969) is investigated using a full three-dimensional hydrodynamic simulations, taking into account self-gravity of the ISM, radiative cooling, and star formation followed by energy feedback from supernovae. This is an essential progress from the previous numerical models, in which 2-D isothermal, non-self-gravitating gas is assumed. We find that the classic galactic shocks appears is unstable and transient, and it shifts to a globally quasi-steady, inhomogeneous pattern due to non-linear development of instabilities in the disk. The spiral patterns consists of many GMC-like dense condensations, but those local structures are not steady, and they evolves into irregular spurs in the inter-arm regions. Energy feedback from supernovae do not destroy the quasi-steady spiral arms, but it mainly contributes to vertical motion and structures of the ISM. The results and methods presented here are a starting point for more consistent treatment of the ISM in spiral galaxies, in which effects of magnetic fields, radiative transfer, chemistry, and dynamical evolution of a stellar disk are taken into account. Subject headings: ISM: structure, kinematics and dynamics — method: numerical